This is important and should be a given. But the more interesting challenge is to highlight the object you’re editing (where your cursor is). It’s not clear even how to exactly visualize it (it could be inside subtract of union of subtract etc).
If you're subtracting cube A from cube B, and you position cube A such that there's no overlap between the cubes, you can't even see cube A. But you can imagine that when I place my cursor in an editor onto the code that generates cube A, that it could be rendered (say transparent), to indicate where it is. You can then more easily position it. Otherwise you have to explicitly render it yourself, or switch between difference and union operators.
